While operating in a cellular radiotelephone system, data messages are continuously transmitted between a radiotelephone and a base station. These messages include orders requesting the radiotelephone transceiver to change transmit power level, to change channel assignment, to release the call, or other similar requests. Some of these data messages are sent on the forward and reverse voice channels. In systems using subaudible data, the messages are interleaved in a continuous stream of data. A system as described in U.S. Pat. No. 4,984,290 (Levine et al.), assigned to the assignee of the present invention, is an example of such a system. Narrowband Advanced Mobile Service (NAMPS) is another example of such a system and is described in more detail in Motorola NAMPS Air Interface Specification Revision D, available from the assignee of the present invention. Sections 2.7.2.2 and 3.7.2.2 of the above specification give a more detailed description of the forward and reverse voice channels.
The data message format used on the narrow forward voice channel (base to radiotelephone) consists of 28 bits of content encoded into a (40, 28) error correcting Bose-Chaudhuri-Hocquenghem (BCH) coded Manchester modulated data message with a distance of five sent at 100 bits/second. The BCH code allows for the correction of two bit errors and is a shortened version of the standard (63, 51) BCH code. Full decoding, allowing two bit error correction of the (40, 28) BCH requires utilization of a complex decoding algorithm such as defined by Berlekamp which requires computation using Galois field arithmetic. S. Lin & D. Costello Jr., Error Control Coding, 141-183, (1983), discusses BCH decoding in more detail. A much more practical and less complex decoder is the error-trapping decoder, which allows for correction of one bit error. For the current description a single bit error trapper is assumed to be used as the BCH decoder.
The forward voice channel message is transmitted after a single transmission of a 30 bit word synchronization pattern sent at 200 bits/second using non-return to zero (NRZ) modulation. Four errors are allowed in the synchronization pattern. Bit synchronization is accomplished by receiving one of seven 24-bit continuously transmitted 200 bits/second Digital Supervisory Audio Tone (DSAT) sequences and maintaining synchronization to it. A (48, 36) BCH coded Manchester modulated data message is used on the narrow reverse voice channel (radiotelephone to base).
The reverse voice channel message is similarly transmitted after a single transmission of a 30 bit word synchronization pattern. Bit synchronization on the reverse voice channel is accomplished with a DSAT sequence similar to that of the forward voice channel.
Instead of multiple repeats, as used in some typical cellular systems, an automatic repeat request procedure (ARQ) is implemented for fading protection. When a BCH code correctable subaudible data message is received, an acknowledgement is sent by the receiving unit to the transmitting unit. The message may be transmitted a total of three times if an acknowledgement is not received by the sending unit.
If the radiotelephone is in a low signal level, multipath, or Rayleigh fading environment, the data message may contain uncorrectable errors when it is received. If the message is received three times with BCH code uncorrectable errors, the message is ignored and no acknowledgement is sent. There is a resulting need to correct these BCH code uncorrectable errors, thereby reducing the message error rate for subaudible data communications.